Time evolution of the boost factor f _HMXB = L _HMXB/ L _MS for a stellar population of N = 10 ⁶ stars, a survival fraction f _sur = 1, and Kroupa, Salpeter, and Chabrier IMFs (left, middle, and right panels, respectively). Dot-dashed curves correspond to polynomial fits of the form given by Equation (4), with parameters given in Table 2.